I'm from Victoria, but I had a job capacity assessment last year. Not for AS though; it was for depression/possible psychosis. It doesn't really matter too much how you present on the day. I mean, I wasn't sobbing uncontrollably or talking to imaginary people or anything like that when I had my assessment. I'm pretty sure I just came across as quiet and shy, and that's all. The assessment is more about the questions they ask you, and the things your doctors/therapists/whatever have written about you. The assessor looks at the information they've been given, asks you how long you've had the condition, if you're on medication, how your day-to-day life is affected, and so on. I don't have a clear memory of the interview, but I'm pretty sure it's all about the facts, and not about your presentation. So just be honest and direct, and it should go ok.
Hope that helps.